Fix a flash of document selection when "oncopy" and "inline on range" behaviors interact

Summary:
Ref T13513. In Safari, do this:

• view a 2-up diff with content on both sides;
• select more than one line on the right side; and
• use your mouse to click "New Inline Comment" in the context menu that pops up.

The mousedown event for the "New Inline Comment" click removes the "copy selection" behavior and creates a flash where both sides of the diff are selected.

This doesn't happen with (most) normal clicks because mouse down on a non-grabbable element clears the document selection.

To avoid this, don't reset the copy selection behavior if the user mouses down on an "<a />". This might not be robust, but seems simple and plausible as a fix.

Test Plan:

• See above.
• Before patch: flash of overbroad selection when clicking "New Inline Comment".
• After patch: no selection flash.

Maniphest Tasks: T13513

Differential Revision: https://secure.phabricator.com/D21254